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The conductance of a single 4,4 bipyridine (44BPD) molecule connected to two gold electrodes is calculated using a density functional theory based Green function method. The atomic geometry of such a molecular junction is constructed from the optimized structure of a gold trimer-44BPD-gold trimer complex. Resonant conduction is the main feature of its transport properties. The magnitude of the transmission coefficient at the Fermi level is determined to be T = 1.01 × 10(-2), which is in excellent agreement with the experimental value. The dependence of the transmission on the Au-N bond length and the torsion angle is also discussed.

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The conductance of a Au/1,4-diaminobenzene/Au molecular junction is investigated using a fully self-consistent ab initio technique which combines the non-equilibrium Green’s function formalism with density functional theory. The transmission coefficient at the Fermi level is calculated to be 0.028, in reasonable quantitative agreement with the recent break junction experiments (0.0064). Transport is mainly through the highest occupied molecular orbital (HOMO) and the HOMO-2 state of the 1,4-diaminobenzene molecule. These states are all π conjugated orbitals formed from the p-orbitals of the two nitrogen atoms and a π-type orbital on the benzene backbone. Our calculations also demonstrate that these frontier molecular states form two σ-type bonds with the s-orbitals of the gold adatoms, which is helpful in reducing the dependence of the junction conductance over the anchoring geometry.

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